1. Technical Field
The present invention relates to a lens system suited to forming an image of an object on an opto-electric converting element, such as a CCD.
2. Description of the Related Art
In accordance with demand for reductions in the size and weight of digital cameras, small-scale zoom lenses for image pickup purposes have been provided. In Japanese Laid-Open Patent Publication No. H04-217219, a zoom lens that has a first lens group with negative refractive power, a second lens group with positive refractive power, and a third lens group with positive refractive power arranged in this order from the object side is disclosed. In the lens system magnification is carried out by moving the first lens group and the second lens group. In the case of a digital still camera (a so-called “digital camera”) that records images using an opto-electric converting element such as a CCD, the dynamic range is narrow and the recorded images are still images. Therefore, compared to a video camera lens and a lens for silver halide photography, in accordance with the characteristics of a CCD, the lens system is designed with the exit pupil position set sufficiently distant from the imaging surface and it is preferable for the image side of the lens system to be made telecentric or to have similar characteristics.
To make the image pickup side telecentric, the retrofocus-type construction with negative-positive-positive powers disclosed by Japanese Laid-Open Patent Publication No. H04-217219 is suitable. However, with the lens system disclosed by Japanese Laid-Open Patent Publication No. H04-217219, although a sufficient image forming performance is achieved, the construction includes as many as ten lenses and the overall length of the lens system is long. The telecentric performance is also insufficient. This is because in order to achieve a sufficient telecentric performance, it is necessary to use a design where the exit pupil position is sufficiently distant from the imaging surface of the element. To do so, it is necessary to sufficiently raise the negative power of the first lens group and to reduce the positive power of the second lens group, but if the negative power of the first lens group is excessively raised, it becomes no longer possible to correct distortion, and if the positive power of the second lens group is excessively reduced, the magnifying action is reduced, the lens length is increased, and the correction of aberration by the second lens group becomes insufficient. Accordingly, the power of the first lens group is suppressed to an extent, the power of the second lens group is maintained, and a large number of lenses are disposed for correcting aberration. Such a design leads the lens system with insufficient telecentric performance and not compact that dose not suit for a digital camera.
By using aspherical lenses, it is possible to reduce the number of lenses and to improve aberration. A variety of aspherical lenses are available, such as molded glass lenses, hybrid lenses, and plastic lenses, but the desired performance cannot be achieved unless errors such as surface precision and eccentricity are small. Precise aspherical lenses are expensive, so that even if the overall number of lenses is reduced, the manufacturing cost of a lens system cannot be reduced. Plastic lenses that are relatively inexpensive also exhibit a high temperature dependency, so that there is the further problem of it being difficult to maintain a uniform performance. In addition, since the effects of the precision of the lenses themselves, environmental effects due to temperature and the like, and effects such as the assembly precision of the lens system are complexly intertwined, it is difficult to achieve the designed performance in an actual lens system that uses aspherical lenses.
For this reason, it is an object of the present invention to provide a zoom lens with favorable aberration performance by combining a small number of lenses, even if an aspherical lens is not used.